The use of animals immunized against angiotensin I and renin to study the role of angiotensin and aldosterone in hypertension of renal origin has raised serious questions about the role of the renin- angiotension-aldosterone system (RAA). We have developed an assay for angiotensin II that permits measurement of free angiotensin in the presence of circulating anti-angiotensin II antibody. Preliminary experiments demonstrate the presence of free angiotensin II in the plasma of animals with high titers of anti-angio II antibody. These observations reopen the question of the importance of RAA system in renovascular hypertension and the present proposal is designed to study the behavior of the RAA system, physiologically and in hypertensive states in the presence of high titers of anti-angiotensin antibodies.